The present invention relates to an adhesive strip containing a temporary fixing adhesive for a first temporary fixing of two substrates to one another as a first stage to a second final fixing of the two substrates, wherein the second final fixing is effected by means of a final fixing adhesive, in particular an epoxy resin, and this final fixing adhesive absorbs the temporary fixing adhesive in the course of the second final fixing, to a use of this adhesive strip and to a method for producing a composite.
Fiber-reinforced epoxy resins have numerous areas of application. For instance, they are used in the automobile industry, in boatbuilding, in aircraft construction or in the production of rotor blades for wind turbines. In the production of rotor blades for wind turbines, many layers of glass-fiber mats are laid out over a large area in the required shape. In a so-called vacuum infusion process, epoxy resins are then drawn in (process names: vacuum-assisted resin transfer molding (VARTM, vacuum infusion). This is shown in FIG. 1. In order to prevent the glass-fiber mats being displaced or forming folds in this process, which constitutes a greater problem the larger the glass-fiber mats are, it is desirable to hold the glass-fiber mats temporarily in the required position. Hence, they are either adhered at the edges or spray adhesives are used. Edge adhesion is inconvenient and susceptible to error when fixing the mats. Since the mats are fixed only at their edges, there may be faults in the mats in regions of the mold lying further inwards, which have a negative influence on the product. The application of spray adhesives is likewise time-consuming and susceptible to error, since metering of the adhesive is incumbent upon the person applying it. A uniform layer thickness of the adhesive layer thus cannot be guaranteed. In addition, spray adhesives contain large quantities of in some cases harmful or irritating solvents, such as for example styrene or butanone, which have to be evaporated before adhesion. Since a foreign material is also introduced into the composite via the spray adhesive, the performance of the cured glass fiber-epoxide composite is in addition somewhat poorer than that of the pure epoxide composite without the addition of a spray adhesive. Pressure-sensitive adhesive strips would indeed be simpler to apply, but foreign materials would remain in the composite as for the spray adhesive, and so the performance of the component would likewise become poorer.
Fibrous mats coated with adhesive, so-called “prepregs” (“preimpregnated fibers”), are known from EP 2 229 421 B1 and are used for producing fiber-reinforced epoxides. The core of the invention is that the adhesive is partly crosslinked, with partial esters of an epoxy resin with an unsaturated carboxylic acid being reacted and partly crosslinked. The adhesive can be swollen and/or solubilized in the epoxy resins of the infusion process.
Prepregs, their production and use are described in detail in WO 2013/107829 A1. They are textile fabrics which are impregnated with a reactive resin. To produce storage life and transportability, the reactive resins are usually partly gelled: that is, the curing reaction is initiated and stopped at an early stage (so-called B stage). A clear increase in viscosity of the resin occurs, which renders the impregnated structure manageable. Prepregs of this type have pressure-sensitive adhesive nature and may thus be laminated together at room temperature. Like adhesive strips, they are usually covered with release liners so that they can be stacked or rolled up. The disadvantage of this prior art is that the partly gelled prepregs have to be stored cooled to prevent the curing reaction from continuing.
EP 2 229 421 B1 solves the problem of short storage life of prepregs in the B stage, in that a textile semi-finished product is provided which is treated with a pressure-sensitive adhesive. The latter is partly crosslinked via a free-radical process. The pressure-sensitive adhesive is used for adhesion on a textile carrier. Both the textile fabric and the partly crosslinked polymer remain at the adhered point in the finished component, as a result of which inhomogeneities are generated in the crosslinking density.
WO 2012/026980 A2 describes the use of spray adhesives having pressure-sensitive adhesive nature to influence the flow behavior of the epoxy resin infused in the VARTM. The commercially available adhesive NuTack® E, NuTack® Blu, NidaTack® NT and 3M™ Super 77™ are mentioned as examples. These spray adhesives reduce the interlaminar shear strength of fibrous composites by more than 8%.
WO 1997/003828 A1 describes fiber-reinforced foams. As described therein, the foam may be provided, for example, with a pressure-sensitive adhesive, to fix the fibrous mats. The problem is that the pressure-sensitive adhesive remains in the cured component as an interfering factor, because it negatively influences the strength thereof.
Therefore WO 2013/060588 A1 proposes using thermoplastic adhesives for temporary fixing of fibrous mats that have the same or a chemically similar resin as the resin used for the VARTM process. Unsaturated polyester resins and vinyl ester resins, which are then dissolved in the VARTM by styrene-containing resins, are described as advantageous.
Crosslinkable adhesives based on epoxides are described in WO 2013/060588 A1 as disadvantageous, since they tend to form a sub-structure which act negatively on the properties of the laminate. However, epoxy resins are preferably used in the VARTM for high-grade components.
A central disadvantage in the current state of the art is the inconvenient fixing of the fibrous mats using adhesives at the edge or spray adhesives. Indeed, pressure-sensitive adhesives are generally described for fixing of, for example, foams to fibrous mats in such infusion processes. However, these foreign materials remaining in the material produce losses in performance which cannot be tolerated in demanding industries, such as for example aviation and space travel or production of rotor blades. A liquid adhesive acquires pressure-sensitive adhesive nature if it brings with it a certain cohesion. Increasing cohesion is achieved in the prior art, for example, with textile fabrics and/or by UV-induced pre-crosslinking (EP 2 229 421 B1) or by the use of thermoplastic adhesives (WO 2013/060588 A1) which are adhesive only at elevated temperatures.